Previously, optional extra cycles (such as the extra cycle incurred when a
branch instruction is taken) were only mentioned in comments, not in the actual
data, with the understanding that an emulator would just silently add them when
necessary. Some operations made those optional cycles mandatory, or impossible,
so we added the "force_cycles" field, meant to signal to emulators that they
should *not* add the extra optional cycles to this operation, but this was
unsatisfying. It meant emulators had to know about this special convention, and
it would be impossible for automated tests to know if they'd covered all the
situations that might incur optional cycles.

Now, instead of "cycles" and "force_cycles" we have "base_cycles" (the absolute
minimum number of cycles an operation takes) and "extra_cycles" (an additional
number of cycles the operation might take under some circumstances. For any
operations with extra_cycles=0, emulators shoul...